Controlling ripening in grapes

Increased temperatures can alter grape berry composition and result in lower priced fruit that has reduced potential for making higher value wine. With current temperatures at optimal or above optimal levels for producing high-quality fruit and wine, grape growers are looking for new ways to control ripening to ameliorate these effects.

“South Australia produces more than half the wine made in Australia each year,” said Dr Christine Böttcher, research scientist with Dr Christopher Davies’ group investigating the control of grape berry at the CSIRO Agriculture Flagship in Adelaide. “Climate change is a big concern for the wine industry because of increasing temperatures and the effect this has on grapes.”

Grape berry development is particularly effected by temperature - the berries develop with less colour, reduced levels of flavour and aroma compounds, and increased sugar levels.

“Increases in temperature are predicted to continue and this will affect the quality of the berries,” Böttcher said.

The ripening hormone

Berry development occurs in three stages. In stage 1 the fruit set and berries begin to grow along with the development of seeds. Stage 2 involves a pause in berry growth and Stage 3 is the ripening phase where berries change colour and soften with accumulation of sugars and a reduction in organic acids.

It is this third ripening stage that is the focus of Böttcher’s work.

A number of plant growth regulators are involved in ripening - abscisic acid which promotes ripening, the auxin indole-3-acetic acid (IAA) that inhibits ripening and ethylene, which has a complicated role in berry development and whose role in ripening is not yet well understood.

“Ethylene is known as the ripening hormone,” said Böttcher.

Climacteric fruits such as tomatoes, apples and pears undergo a rise in cellular respiration associated with increased ethylene production upon fruit ripening. Ethylene seems to be the key in controlling ripening in climacteric fruit.

Non-climacteric fruits, such as grape and olives, produce very small amounts of ethylene - a transient increase of ethylene production occurs just before the onset of ripening (called veraison in grapes) - but this is not associated with an increase in respiration and these fruits do not respond to ethylene treatment in the way that climacteric fruits do.

How to control ripening

Böttcher has been analysing the effects of a synthetic auxin, 1-napthalene acetic acid, the ethylene-releasing substance Ethrel and another compound, aminoethoxyvinylglycine (AVG), to unravel how ripening is controlled in grapes.

“AVG inhibits ethylene biosynthesis and has the opposite effect to Ethrel when applied to pre-veraison berries,” said Böttcher. “It advances ripening, whereas Ethrel treatments can lead to ripening delays.

“Ethylene levels in grapes are very low, so they are very difficult to measure,” Böttcher continued. “Therefore, we have been looking at the expression of genes involved in ethylene biosynthesis and perception.”

When AVG was added to cultured berries, Böttcher and colleagues found that the expression of genes encoding receptors for ethylene was decreased, a response typically seen when ethylene levels are reduced.

“The inhibition of ethylene biosynthesis by AVG provides a possible explanation for the opposite effects of Ethrel and AVG treatments on grape berry ripening,” Böttcher summarised.

The twist

But there is a twist. AVG also inhibits the biosynthesis of IAA, the auxin that inhibits ripening, whereas Ethrel application increases IAA accumulation in berries. Therefore, changes in grape berry ripening elicited by Ethrel or AVG application might in fact be IAA-mediated.

Surprisingly, the researchers found that applying Ethrel a week before veraison advanced ripening, which was counter to what they expected because of the presumed increase in IAA levels by Ethrel treatment.

“We then looked at the developmental expression of genes encoding enzymes involved in the biosynthesitic pathways of IAA and ethylene and found expression patterns indicative of an ethylene-induced increase in IAA production at around the time of ripening initiation,” said Böttcher.

IAA levels are low at this stage of berry development, but a conjugate of IAA with aspartic acid (IAA-Asp) rapidly accumulates, not just in grapes, but also in ripening tomatoes.

“The IAA-Asp conjugate has become a focus of our research as it might represent an ethylene-induced ripening signal in both climacteric and non-climacteric fruit,” she said.

Böttcher presented the Australian Society for Plant Scientist 2014 Functional Plant Biology Best Paper Award Lecture at the 2014 ComBio meeting entitled ‘Auxins or ethylene - who controls grape berry ripening?’